Classical "antimutator" DNA polymerases of bacteriophage T4 were examined for their effects upon frameshift mutation rates at a number of positions within rII cistrons. Their antimutagenic activities reduced frameshift frequencies at a number of sites, but at other sites the opposite occurred: the mutant polymerases exhibited clear mutator activities. This dichotomy reveals the operation of two distinct mechanisms of frameshift mutagenesis that are correlated with the DNA sequences at the frameshift sites. Frameshift mutants subject to the antimutator effects of the mutant polymerase lie in A: T-run DNA sequences, where mutations presumably arise by means of the interstrand DNA misalignments postulated by classical theory. The frameshift mutants produced by the mutator activity of these same polymerases lie in quasipalindromic DNA sequences, where mutations are postulated to arise by aberrant metabolism of DNA secondary structures such as hairpins.